Animated FBX Export Glitch - Foot Bones Jerking
I thank God for showing me the solution! All you have to do is add a "false foot" bone that is NOT a deform bone to each leg. I named this bone FootParent. 

Connect this bone to the Lower_Leg bone (which has an IK constraint).  Set FootParent's Inherit Rotation and Inherit Scale to off, and add a Copy Rotation constraint which copies your Foot_IK bone (X + Y invert, Target: Local With Parent, Owner: Local).

Now duplicate this FootParent bone (or add a new one in the exact same location and orientation) and name it Foot. This will be your deform bone, and is NOT connected to anything.

Finally add 3 constraints to your Foot bone: Copy Location of the FootParent bone (Target: Local With Parent, Owner: Local); Copy Rotation of your Foot_IK bone (X + Y invert, Target: Local With Parent, Owner: Local), and optionally Copy Scale of your Foot_IK bone (Target: Local, Owner: Local). Now all your animations will export to FBX the way you created them in Blender!